1 Field of the Invention
The present invention concerns a multiuser detection method and device. More particularly, the present invention concerns a maximum likelihood multiuser detection method and device for a DS-CDMA (Direct Sequence Code Division Multiple Access) telecommunication system.
2 Description of Related Art
In a DS-CDMA mobile telecommunication system, the separation of the communications coming from or going to the different users is achieved by multiplying each complex symbol of a user by a spreading sequence which is peculiar to him, also referred to for this reason as the user signature. The spreading frequency (chip rate) being greater than the frequency of the symbols, the signal transmitted by each user is distributed (or spread) in the space of the frequencies. The ratio between the band occupied by the spread signal and the band occupied by the information signal is referred to as the spreading factor. On reception, the separation of a given user is obtained by means of a filtering adapted to the corresponding signature. When the transmission channel has a plurality of propagation paths, the output of the adapted filtering contains as many correlation peaks. Each path of the channel can be modelled by a complex multiplicative coefficient and a delay. The signals being propagated along the different paths can be aligned and combined by means of complex coefficients which are conjugates of the path coefficients, thus effecting a filtering adapted to the transmission channel. In order to simplify the terminology, the general expression “filtering adapted to the user k” will encompass both the filtering operation adapted to the signature of the user k and the filtering operation adapted to the transmission channel.
To combat the interference between signals destined for (the downlink) or coming from (the uplink) the different users, multiuser detection methods have been proposed, and notably iterative detection methods such as those known as PIC (Parallel Interference Cancellation) and SIC (Serial Interference Cancellation). They are based on the iteration of an interference elimination cycle including the estimation of the symbols transmitted, the evaluation of the interferences and their subtraction from the signals received. Although of high performance, these methods are not optimal since they do not provide an estimation in the sense of the maximum likelihood of the symbols transmitted by the different users.
A method of multiuser detection with maximum likelihood inspired by the Viterbi algorithm was proposed by S. Verdu in an article entitled “Minimum probability of error for asynchronous Gaussian multiple access channels”, published in IEEE Transactions on Information Theory, pages 85–96, January 1986, but its complexity is prohibitive since it varies exponentially with the number of users.
More recently a method of multiuser detection with maximum likelihood using a representation by a lattice of points was proposed by L. Brunel et al, in an article entitled “Euclidian space lattice decoding for joint detection in CDMA system” published in Proceedings of ITW, page 129, June 1999. According to this method, a vector characteristic of the received signal representing a statistic sufficient for the maximum likelihood detection of the symbols transmitted by the different users is determined. It is shown under certain conditions that the characteristic vector can be represented as the point in a lattice disturbed by a noise. The detection then consists of seeking the point in the lattice closest to the point corresponding to the vector received. However, the dimension of the lattice to be used generally being 2·K where K is the number of users, the number of points to be tested is still very high.